It seems the "restore" cheatcode would help here. In the DsL FAQ Section 2.3 "How Do I Save My Settings?"
there is an explanation of how the saving mechanism in DsL works.
Without testing I'm not sure where the plugins are stored, and if the location is included in /home/dsl/.filetool.lst or not. Your question makes me a bit curious to find out though. I'll post more after a few tests.
So I setup the backup/restore under DSLpanel for a location of hda1, because I had an empty hard drive laying around, but you may want to use one of the others for a USB drive. After that was setup, I installed flash7 as a non-root user so it would be installed in /home/dsl/.mozilla. I also set a bookmark since the FAQ mentioned that specifically. Tested things were installed correctly and working fine before I rebooted.
During the shutdown process, DsL saved the backup file to hda1 as I had set in the DSLpanel. When faced with the boot: prompt I added restore=hda1 to my list ( dsl restore=hda1 ). During bootup I noticed the backup file being restored from hda1. Once DsL was up, I fired up Firefox and every thing worked just as before the reboot.
So that's your answer...